The present invention relates to current mirror circuits. More specifically, the invention relates to a current mirror circuit that is used in a signal processing circuit of audio equipment, video equipment, etc., and is operable with a low power supply voltage.
FIG. 5 shows an example of a conventional current mirror circuit, in which pnp transistors are employed on the side of a positive power supply voltage V.sub.cc. This circuit consists of transistors Q.sub.1 and Q.sub.2 and resistors R.sub.1 and R.sub.2. The emitter of the transistor Q.sub.1 is supplied with the voltage V.sub.cc via the resistor R.sub.1. The base and collector of the transistor Q.sub.1 are not only connected to each other but connected to an input terminal. The emitter of the transistor Q.sub.2 is supplied with the voltage V.sub.cc via the resistor R.sub.2. The base and the collector of the transistor Q.sub.2 are connected to the base of the transistor Q.sub.1 and an output terminal, respectively.
Since the bases of the transistors Q.sub.1 and Q.sub.2 are interconnected and therefore at the same voltage, when the transistors Q.sub.1 and Q.sub.2 are in an active state, a voltage drop from the power supply terminal to the base of the transistor Q.sub.1 is equal to a voltage drop from power supply terminal to the base of the transistor Q.sub.2. That is, a voltage drop across the resistor R.sub.1 plus 1Vf (voltage corresponding to the base-emitter internal potential barrier) is equal to that of a voltage drop across the resistor R.sub.2 plus 1Vf.
Therefore, the relationship between respective currents flowing through the resistors R.sub.1 and R.sub.2 is determined in accordance with resistance values of the resistors R.sub.1 and R.sub.2. In particular, if such resistance values are made equal to each other, the two currents flowing through the resistors R.sub.1 and R.sub.2 become substantially the same.
As a result, apart from a difference between very small, negligible current components, it can be said that an output signal current I.sub.out flowing from the output terminal to a circuit of the following stage or a load is the same as an input signal current I.sub.in flowing from the, input terminal to a circuit of the preceding stage. If only a current variation component is particularly selected as the signal current, the input signal current and the output signal current will coincide with each other. For the above reasons, this type of circuit is now widely used to invert the direction of the signal current.
As described above, in the conventional current mirror circuit, the voltage drop from the power supply terminal to the base voltage of the transistor Q.sub.1 is a sum of the voltage drop across the resistor R.sub.1 and 1Vf. Since the input terminal is directly connected to the base of the transistor Q.sub.1, a voltage drop from the power supply terminal to the input terminal also takes the same value.
The above voltage relationship means that if an input signal voltage exceeds the power supply voltage V.sub.cc minus the above voltage drop, the direction of the signal current cannot be inverted properly, that is, the above circuit does not operate normally. In other words, the power supply voltage V.sub.cc should have a margin of not less than the voltage drop across the resistor R.sub.l plus 1Vf with respect to the effective signal voltage.
However, with the conventional current mirror circuit, which is based on the power supply voltage having enough margin, the circuit design of equipment is now in a very difficult situation because of the recent requirements in connection with the equipment down-sizing, specifically the requirement that portable equipment having a small battery be kept operable for a long time, and because of such limitations on the circuit design as a reduction of the breakdown voltage due to the miniaturization of IC patterns.